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A measure of the velocity of an astronomical object along the observer's line of sight. The radial velocity is determined by comparing the wavelengths of lines in the object's spectrum with their laboratory values, the differences being due to the Doppler shift. Corrections are usually applied for the Earth's rotation on its axis and its orbital motion around the Sun. By convention, velocities away from us (redshifts) are positive; those towards us (blueshifts), negative. See also redshift.